Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The invention relates to a method and apparatus for measuring an analyte in blood and other tissue. The method employs Raman spectroscopy wherein a temperature probe within the tissue is excited with electromagnetic radiation. The method results in enhanced spectral information related to concentration of the analyte.
There has long been considerable interest in the non-invasive monitoring of body chemistry. There are 16 million American diabetics, all of whom would benefit from a method for non-invasive measurement of blood glucose levels. Using currently accepted methods for measuring blood glucose levels, many diabetics must give blood five to seven times per day to adequately monitor their insulin requirements. With a non-invasive blood glucose measurement, closer control could be imposed and the continuing damage, impairment and costs caused by diabetes could be minimized.
Blood oximetry is an example of an application of electronic absorption spectroscopy to non-invasive monitoring of the equilibrium between oxygenated and deoxygenated blood (U.S. Pat. No. 5,615,673, issued Apr. 1, 1997). Similarly, vibrational spectroscopy is a reliable mode of quantitative and qualitative ex vivo analysis for complex mixtures, and there are reports of in vitro applications of this method to metabolically interesting analytes (S. Y. Wang et al., 1993, Analysis of metabolites in aqueous solution by using laser Raman spectroscopy, Applied Optics 32(6):925-929; A. J. Berger et al., 1996, Rapid, noninvasive concentration measurements of aqueous biological analytes by near-infrared Raman spectroscopy, Applied Optics 35(1):209-212). Infrared measures, such as vibrational absorption spectroscopy, have been applied to skin tissue, but with success limited by unavailability of suitable light sources and detectors at crucial wavelengths, and by heating of the tissue due to the absorption of incident radiation (U.S. Pat. No. 5,551,422, see also R. R. Anderson and J. A. Parrish, 1981, The Optics of Human Skin, J. Investigative Dermatology 77(1):13-19). Previous attempts to provide methods for non-invasive blood glucose monitoring are summarized in U.S. Pat. No. 5,553,616, issued on Sep. 10, 1996.
The invention provides a method and apparatus for measuring an analyte in a tissue of a subject. The method uses Raman spectroscopy and radiation with a wavelength selected to excite a temperature probe within the tissue. The temperature probe and the analyte are sufficiently proximate to one another that energy deposited into one by absorption of radiation is transferred to the other. This approach permits an enhancement of the signal-to-noise ratio in the emitted Raman spectra so that spectral changes associated with the concentration of analyte in the tissue can be observed and measured. The signal-to-noise ratio is improved by techniques that enhance the signal, reduce the noise, reduce background, and/or unravel spectral congestion. In one embodiment, excitation of the analyte in addition to the temperature probe allows one to measure analyte concentration by detecting the temperature increase effected in the temperature probe. In addition, measuring the anti-Stokes spectra emitted by the analyte following excitation of the temperature probe avoids background fluorescence. In preferred embodiments, the method comprises contacting the tissue with electromagnetic radiation having a first excitation wavelength, wherein the first excitation wavelength is substantially equal to an absorption wavelength of a temperature probe within the tissue. The Raman spectra emitted by the tissue are collected and analyzed to determine a concentration of analyte present in the tissue. In one embodiment, the analyzing comprises measuring the Raman spectra associated with the temperature probe. The method can further comprise simultaneously contacting the tissue with electromagnetic radiation having the first excitation wavelength and with electromagnetic radiation having a second excitation wavelength, wherein the second excitation wavelength is substantially equal to an absorption wavelength of the analyte, and wherein the analyzing comprises comparing the spectra emitted in response to the first excitation wavelength in the presence and in the absence of the second excitation wavelength.
In one embodiment, the concentration of analyte is determined in accordance with the following relationship:
CA=xcex94T/(CpIAxcex5Axcex94t);
wherein CA is the concentration of analyte;
wherein xcex94T is the temperature shift between the temperature, in Kelvin, associated with Raman scattering emitted by the temperature probe in response to the first and second excitation wavelengths, and the temperature associated with Raman scattering emitted by the temperature probe in response to the first excitation wavelength in the absence of the second excitation wavelength;
wherein Cp is the heat capacity of the tissue; wherein IA is the intensity of the electromagnetic radiation of the second excitation wavelength;
wherein xcex5A is the absorption coefficient for a molecule of analyte;
and wherein xcex94t is the duration of contact with the second excitation wavelength.
In another embodiment, the analyzing comprises measuring the Raman spectra associated with the analyte. The Raman spectra can comprise anti-Stokes spectra.
The method can be performed while the tissue is replete with blood and while the tissue is depleted of blood. In this embodiment, the analyzing comprises determining the difference between the Raman spectra collected in the blood replete and blood depleted states.
Examples of a temperature probe include, but are not limited to, hemoglobin, carboxyhemoglobin, myoglobin, melanin, and bilirubin. Examples of an analyte include, but are not limited to, glucose, creatinine, pyruvate, drugs, blood gases and urea.